Boron carbide is the lightest technical ceramic material and is extremely hard. Boron carbide is considered to have a chemical formula of B4C, although most materials are slightly carbon deficient. Boron carbide is chemically inert, even to most acids, and has a high neutron absorbing cross section. Boron carbide can be hot-pressed into useful shapes for components with outstanding resistance to abrasive wear. B4C can be polished to a mirror finish. B4C has high elastic modulus (>435 GPa), high melting point (2450° C.), and high Hugoniot elastic limit (>18 GPa). Because of this unique combination of properties, B4C is an ideal candidate for many industrial, military and energy applications. B4C is used as a grinding medium for hard materials, in wear resistant sandblasting nozzles, in fast-breeder reactors and high-temperature thermoelectric conversion units, and as a lightweight ceramic armor material.
B4C has a hardness of ˜30 GPa Vickers hardness, which is exceeded only by cubic boron nitride, ˜48 GPa and diamond, ˜115 GPa. Unlike diamond, B4C has a low thermal conductivity but a high thermal stability. Cubic boron nitride (cBN) has a high thermal stability but is only about half as hard as diamond. Major research efforts are directed towards development of super-hard materials with hardness above cBN. Because of their similar atom sizes in carbon and boron nitrides, it has been rationalized that synthesis of phases containing all three elements B, C, and N would yield materials with high hardness and other beneficial properties. A ternary phase B-C-N system, specifically a cubic BC2N phase (cBC2N), has been prepared that has a Vickers hardness of ˜76 GPa. The super-hard materials other than diamond, which is not stable under the conditions required, are attractive for high-speed cutting and polishing of ferrous alloys.
Modification of boron carbide is of interest with respect to augmenting or changing the properties of B4C. Boron carbide is a semiconductor when the stoichiometry is B>4C. The mildly p-type boron carbide, B5C, prepared as a thin film by plasma enhanced chemical vapor deposition (PECVD), becomes an n-type semiconductor when doped with nickel. (Hwang et al., Nickel Doping of Boron Carbide Grown by PECVD, J. Vac. Sci. Technol. BI, 1996 14(4), 2957) In like manner, cobalt, iron, and manganese B5C films have been prepared by PECVD. (Liu et al., The Local Structure of Transition Metal Doped Semiconducting Boron Carbides J. Phys. D: Appl. Phys., 2010 43 024513) Zirconium doped B4.3C semiconductor was prepared by hot pressing under a nitrogen atmosphere of a mixture of boron carbide powder (B4.3C) and Zr nanocrystals at 0.5 atom %, to yield a mixed composition of B4.3C, (BN)4H, and (ZrB2)3H by XRD analyses. (Liu et al., Structural Changes of Boron Carbide Induced by Zr Incorporation, Journal of Materials Science 200 35 387) Hot pressing of B4.3C with nickel at 0.5 atom % under the same conditions resulted in a doped structure with Ni incorporated in the B4.3C crystals, as revealed by XPS and SPM analysis. (Liu, Conductivity Transition of Semiconducting Boron Carbide by Doping, Materials Letters 2001 49 308) No characterization of transition metal doped materials other than their semiconducting behavior has been disclosed.
Hence, there is a need for a super-hard material that is relatively inexpensive, with the hardness of cBN or better. Modification of B4N to achieve a super-hard material would be advantageous.